1. Field of the Invention
This invention relates to an optical disk and optical information recording/reproduction apparatus, and particularly to an erasable optical disk on which information is recorded, reproduced and erased repeatedly through irradiation of a laser beam, and to an optical information recording/reproduction apparatus which records and reproduces information on the erasable optical disk.
2. Description of the Related Art
FIG. 8 shows, as an example, the phase transition between the noncrystalline state (A) and the crystalline state (C) of a phase varying recording medium on a conventional optical disk. The recording medium records a signal by a variation between the noncrystalline state (A) of a small reflectivity and the crystalline state (C) of a large reflectivity. A signal is recorded in such a way that a portion of the recording medium in the crystalline state (C) having a large reflectivity is heated locally to a temperature near the fusing point and then cooled quickly to bring it into the noncrystalline state (A) having a small reflectivity. The recorded signal is erased by heating the recording medium to a temperature near the fusing point and then cooling it slowly so that the portion irradiated by the laser beam is brought into the crystalline state (C) having a large reflectivity.
FIGS. 9A and 9B are diagrams illustrating the principle of signal recording and erasing by using a recording/reproduction laser beam and an erasing laser beam. FIG. 9A shows the spot shape of laser beams for realizing the heating and rapid cooling condition and the heating and slow cooling condition for the recording medium. FIG. 9B shows the respective laser intensity distributions. In these figures, indicated by 23 is a spot of a recording/reproduction laser beam of a short longitudinal length of the order of 1 .mu.m, 23a is its laser intensity distribution, 24 is a spot of an erasing laser beam of a long longitudinal length in the range from several .mu.m to ten-odd .mu.m, and 24a is its laser intensity distribution. Indicated by 25 is a guide track on which the recording medium is deposited by evaporation. Depending on the differences in the laser spot longitudinal length and intensity distribution, the two distinct heating and cooling conditions are determined, i.e., the short longitudinal length beam spot determines the heating and rapid cooling condition, and the long longitudinal length beam spot determines the heating and slow cooling condition.
In operation, the erasing laser beam 24, which precedes the recording/reproduction laser beam 23 irradiates the information section on the guide track 25 with a constant intensity so that an old signal recorded in the information section is erased and then the following recording/reproduction laser beam 23 records a new signal in the information section.
With the above-mentioned structure, however, it is necessary that the recording/reproduction laser beam 23 and the erasing laser beam 24 are positioned on the same guide track 25. In a two-beam optical information recording/reproducing apparatus generally in use, two separate semiconductor laser sources are used to produce two respective laser beams 23 and 24 which are arranged as shown in FIG. 9A. Hence, it is difficult to maintain both laser beam spots 23 and 24 positioned on the same guide track only by the precision of mechanical structure. Accordingly, it becomes necessary to apply tracking servo control to the erasing laser beam 24 in the same way as the recording/reproduction laser beam 23. However, even when the tracking servo control is applied to the erasing laser beam 24, the tracking actuator provides a movable range of 2-3 .mu.m in the disk radial direction. Therefore, even when the tracking servo system is in operation, it is not guaranteed that the erasing laser beam 24 tracks the same guide track 25 in the same way as the recording/reproduction laser beam 23. On this account, there has been a problem that the erasing laser beam erroneously erases the record on an adjacent track. Besides, the same problem has been the case where the laser beam brings about track jump due to the vibration or shock.